Hydroxyl rotational temperatures and intensities in the nightglow

Planet. Space Sci. 1967. Vol. 15, pp. 1515 to 1523.

Pergamm

Pms

Ltd.

Pfinted in Northern Ireland

HYDROXYL ROTATIONAL TEMPERATURES INTENSITIES IN THE NIGHTGLOW

AND

G. J. KVIFTE The Auroral Observatory, Tromss, Norway (Received 25 April 1967)

Abstract-Spectra of several bands of the OH nightglow emission are obtained at Troms0 and As during the autumn 1963 and spring 1964, respectively. Rotational temperatures have been derived, giving an average of 210 f 10°K at both stations. No seasonal dependence or latitude effect is found. OH temperatures obtained by earlier investigators are discussed and found to agree mutually and with those reported here, assuming the OH emission height near 100km and the atmospheric temperature at this height to vary with solar activity. Absolute band intensities are computed, giving mean values in agreement with earlier investigations. At both stations the observations seem to indicate a seasonal variation of the intensity, showing a minimum during winter. Any latitude effect should be small, possibly giving less intense OH bands north of 60”N. 1. INTRODUCTION

Since the discovery of the rotation-vibrational OH bands in the night-glow by Meinel (1950a), these emissions have been studied by a number of observers. As the lines of the P-branches of these bands usually are well separated, as observed by a spectrometer of reasonably good dispersion, rotational temperatures can easily be obtained from the intensince the equivalence of the rotational OH sity distribution of these lines. Moreover, temperature and the temperature of the emitting atmosphere seems well established (see e.g. Wallace, 1962), most workers have concentrated on this aspect of the OH investigations. In this paper will be presented OH temperatures and intensities obtained from stations at 60”N and 70”N in Norway during the winter 1963-64. The variation with season and latitude will be discussed, and the results compared with earlier work. 2. PREVIOUS WORK A review of work previous to about 1960 is given by Wallace (1962). Some main results will, however, be given here, also including later works, as some background is needed for

comparison with the results to be presented. The temperatures and intensities obtained by the different authors show rather diverging results. This may in part be due to the fact that the observations are carried out with different equipment and techniques and at different times. Several systematic studies have been carried out to find any seasonal variation of the rotational temperature. Among others Wallace (1961) and Krassovsky et al. (1961) have found a maximum in winter and a minimum in summer. While Wallace at 42”N reports extreme values of about 230°K and 200”K, respectively, Krassovksy et al. find corresponding values near 300°K and 220°K at 62”N. McPherson and Valiance Jones (1960) and G. Kvifte (1961), however, have found no such variation with season at latitudes from 52”N to 75”N. Up to 1964 all available data seemed to indicate a variation of the OH rotational temperature with latitude, the temperature decreasing from around 290°K at equator to a nearly constant value of about 220°K from 40”N to say 60°N, followed by an increase to more than 1515

1516

G. J. KVIFTE

300°K towards the pole (cf. G. Kvifte, 1961). Later, however, Noxon (1964) from a jet aircraft found the OH-temperature to decline from 270°K at 15”N through 210°K at 60”N to a value below 180°K at 75”N. Observations have given intensities for a number of OH bands extending from the green to far in the infrared. The absolute values show a considerable scatter, which is not surprising when the difficulties encountered in such determinations are taken into account. The relative intensities seem, however, to be mutually consistent. Several investigators have found the OH intensity to depend on season. Barbier (1959), Dufay (1959) at 44”N and Krassovsky et al. (1961) at 62”N report a winter maximum during the months November-January and a minimum during summer in June-August. The maximum intensities are usually found at least a factor 2 greater than those obtained during summer. However, authors like Nakamura (1961) at 35”N and G. Kvifte (1961) at 60”N have not found any variation with season at all. Even more diverging results are obtained when investigating the dependence of intensity with latitude. Barbier (1957) has found a tendency towards increasing intensities with latitude at low latitudes. In a private communication to Chamberlain and Meinel (1954) Oliver reports almost constant values at three stations situated from Puerto Rico in the south to the northern part of Greenland. Noxon (1964), however, finds decreasing intensities with latitude, at least from middle latitudes towards the north pole. Several methods have been used to establish the altitude of the emitting OH layer. The most reliable results are probably those obtained from observations made from rockets passing through the layer. A rocket experiment by Packer (1961) places the height of maximum emission between 80 and 90 km, while Baker (1966) finds a broad maximum extending from 95 km to 100 km. Earlier investigators, mostly using less precise methods like triangulation or the van Rhijn method, have usually found heights below 80 km. 3. OBSERVATIONAL TECHNIQUE

The observations were carried out with the fast scanning photoelectric grating spectrometer SP3, which is shortly described by Omholt et al. (1962). The photomultiplier used was an EMI tri-alkali cell, cooled with solid CO, to reduce the dark current to a minimum. The recording system is based on photon-counting with a ratemeter and servorecorder. The count rates are punched out at appropriate time intervals with an accuracy of 1 per cent at full scale. Subsequent sweeps can then directly be added by an electronic computer to give more reliable spectra. The observations were made at the Aurora1 Observatory, Tromss (70’N) during the period September-November 1963 and at the Agricultural College of Norway, As (near Oslo, 60”N) from February to April 1964. Although the period of observation was near to sunspot minimum, aurora frequently disturbed the nightglow measurements in Tromsar. Moreover, bad weather conditions did often obstruct the investigations there. As a result only 7 nights were used for the OH-observations from Tromso. At As successful observations were obtained during 12 nights. To combine the requirements of high resolution and high light power, the instrument was used with a slit width equivalent to a resolution of about 5 A. The region to be scanned was set to an interval of 100-200 A around the OH-band to be analyzed, and a scanning time of 4 min was used. The instrument was pointing in different azimuth angles during the observations, and was usually directed about 20” above the horizon in order to utilize the higher intensity at

NIGHTGLOW

OH ROTATIONAL

TEMPERATURES

AND INTENSITIES

1517

large zenith angles. In spite of these precautions, data had to be added from at least 1 hr’s observations to achieve spectra able to give a reasonably accurate temperature determination. To make sure that aurora did not disturb the OH-observations, a photometer recording the intensity of the green oxygen line 5577 A was directed towards the same part of the sky as the spectrometer. The absolute intensities were obtained through calibration with a tungsten-lamp, which had a known intensity distribution, and portable low brightness sources as secondary standards. These standards contain a phosphor excited by p-radiation from tritium. 4. ROTATIONAL TEMPERATURES

The method used for determining rotational temperatures is that described by G. Kvifte (1961). When thermal equilibrium exists, the intensity I(J), in photons/set, of a line of a rotation-vibration band is given by: I(J) = C . S(J). G(J) . exp [--F(J)hc/kT]

(1) Theoretical expressions for the line strengths S(J) are given by Honl and London (1925), while the wave numbers Y(J) and the rotational term values F(J) are given by, among others, G. Kvifte (1960). As the P-lines are well separated by our instrument, intensities of the individual lines of the P-branches can be obtained. Knowing these quantities, a diagram can be constructed containing the plots of In [I(J)/S(J) . ~~(41 a gainst F(J)hc/k for the different P-lines of each OH band to be analyzed. According to equation (1) the temperature can then be obtained from the slope of the straight line best fitting the data. To establish this line and the limits of uncertainty, the method of least squares is used. The temperatures obtained in this investigation are shown in Table 1. 4.1. Seasonal variation of temperature

As seen from Table 1, data from a few nights only were used in computing rotational temperatures. The results cover, however, most of the periods of observation and are relatively accurate. With observational periods of almost 2 months at both stations the suspected dependence of the OH temperature with season should be detectable. We see, however, that with one exception the temperatures are well grouped around their mean values, which are found to be 210 f 10°K both at Tromso and As, and no tendency is found of any variation with time. The unusual high temperature of 260 f 25°K from As, which is not used in computing the mean value there, is thought to be caused by a more accidental event. The results obtained are in agreement with the work of G. Kvifte (1961) and are also confirmed by his latest measurements from As during the period January-February 1964 (private communication). On the other hand these results disagree with those of Krassovsky et al. (1961) who report a maximum near 300°K in January-February at high latitudes. The observations reported here, however, do not preclude a short winter maximum in December-January, when no observations were obtained. If this is the case, our values will still be in conflict with the results of Krassovsky et al., as their maximum was broader than 2 months and in addition displaced about a month into the period of observation at As. The dependence with season found by Wallace (1961) at medium latitudes agrees reasonably well with the variation of the atmospheric temperature at OH-heights as given by Murgatroyd (1965). This may indicate a constant height level, within some kilometers, of the emitting OH layer throughout the year at these latitudes. The results reported in this

G. J. KVIFTE

1518 TABLE 1. OH ROTATIONAL Observing station Tromss

Date Sept. 23, 1963 Sept. 30 Oct. 1 Oct. 4

Oct. 18 Nov. 11 Nov. 18 AS

Feb. 18, 1964 Mar. 7 Mar. 8 Mar. 10 Mar. 13 Mar. 15 Mar. 16 Apr. 1 Apr. 2 Apr. 3 Apr. 5 Apr. 13

TEMPERATURES

Local time

AND

ABSOLUTE

INTENSITIES OBTAINED

Number of Band measured scans

0230-0315 0120-042.5 1920-2205 0045-0410 2005-2055 2055-2245 0310-0410 2205-2255 0400-0640 0145-0320 0145-0640 2115-0030 2115-0340 2100-2225 2320-0430 223C.0110 0130-0455 2055-2255 01 l&O430 0018-0440 2320-0130 0130-0430 0230-0415 2240-0240 2310-0110 01 lo-0340 2315-0120 0155-0350 2150-0300

9 31 26 30 12 21 12 13 23 21 61 26 45 21 38

6-l 9-3 9-3 8-2 8-3 8-3 6-l 6-l 8-3 9-4 9-4 6-l 6-l 6-l 6-l

32 30 30 30 38 23 29 24 38 29 29 31 20 58

6-l 1 9-4 9-4 9-4 8-3 8-3 6-l 6-l 8-3 8-3 8-3 8-3 9-4

Rotational temp. (“K)

2OOilO 218 i 10 216 & 20 208 i 10 214 & 15 220 rt 25 260 i

25

205 i -

15

200 i -

10

215 f 7 212 * 7

IN NORWAY 1963-1964

Absolute intensity CR) 110 84 92 50 255

255 115 92 275 540 480 66 66 87 92 103 93 920 900 980 420 455 112 107 510 540 540 550 1080

Remarks Hazy Faint aurora

Moon Faint aurora Faint aurora Faint aurora

Mean

temperature at Troms0: 210 i 10°K. Mean temperature at .b: 210 i 10°K. paper are not inconsistent with this assumption, since the atmospheric temperature variation during winter at OH heights at latitudes between 60”N and 70”N does not exceed the limits

of uncertainty in the data. The values of Krassovsky et al., however, do not fit in with this picture. A possible explanation may be that their increase, which is found from one winter’s observations only, is caused by some other effect. As will be suggested in the discussion given at the end of the next section, the increasing activity of the Sun during their period of observation may explain their results. 4.2. Latitude variation of temperature As the observations were carried out with the same instrument and techniques at the two stations, a variation in temperature with latitude between 60”N and 70”N should be detectable. No correction should be necessary to reduce the data to a common time of observation as no, or at least a very small, temporal variation of the temperature was found at either of the two stations. As is seen from Table 1, the data show essentially the same temperature at the two latitudes. Any difference in OH temperature at 60”N and 70”N this winter should at most be lo-15°K. This supports the assumption of a constant OH height with latitude as the variation of the atmospheric temperature at OH heights is of this magnitude.

NIGHTGLOW

OH ROTATIONAL

TEMPERATURES

AND INTENSITIES

1519

The mean temperature from As is in good agreement with previous work at this and lower latitudes. The Tromso value, however, is considerably lower than most authors find at corresponding latitudes, with the one exception of Noxon (1964) who reports temperatures well below 200°K. This discrepancy might be caused by the fact that the observations were made at different times of the year. To investigate this possibility the observed temperatures are reduced to their winter solstice values in the following way: The OH layer is assumed to be at a constant height of about 100 km throughout the year and at all latitudes. This height may be a little too great according to earlier determinations, but it fits well with the conclusions to be drawn here. Moreover, the rotational temperature is supposed to vary with season as the atmospheric mean temperature given by Murgatroyd (1965). The rotational temperatures at winter solstice, extrapolated under the given assumptions, are given in Table 2 and Fig. 1 together with the original data for comparison. As is seen, TABLE 2. LATITUDE VARIATIONOF OH TEMPERATURES

Author

Date

Blackwell ef al. Noxon

1960 1964

Fedorova Wallace Wallace Wallace Meinel Cabames et al. Dufay J., Dufay M. Dufay M. Nguyen-Huu-Doan

(1) 1961 1961 1961 1950b 1950 1951 1959 1963

Mironov et al. Gush, Valiance Jones McPherson, Valiance Jones Produkina Shuyskaya Noxon McPherson, Valiance Jones Kvifte G. Kvifte G. The author Noxon Krassovsky et al. Krassovsky et al. Mironov et al. Noxon Produkina The author Noxon McPherson, Valiance Jones Chamberlain, Oliver Noxon Noxon (1) (2) (3) (4)

Aug. 1958 Apr. 1964

Latitude (“N)

295 275 f 35

295 275

Jan: 1950 Aug. -Jan. 1949 Winter 1949-50 Nov. 1957 Jan.

40 43 43 43 43 44 44 44 44

215 zt 15 (2) 220 * 5 205 zt 5 225 & 5 240 f 5 (2) 185 f 45 (2) 240 f 7 (2) 240 (3) 230

215 220 215 225 240 195 240 245 230

1958 1955 1960 (1) (1) 1964 1960

Winter 1956-57 June Feb.-May 1958 Mar. 1963 Mar. 1958

51 52 52 56 57 55-62 59

215 + 20 (2) 200&20 215 f 25 220 + 20 (2) 250 i 10 (2) 195 i 15 230 (2)

220 210 225 220 250 220 255

1961 1961 1964 1961 1961 1958 1964 (1)

Jan.-Mar. 1958 Jan.-Mar. 1959 Feb.-Apr. 1964 Mar. 1964 Winter 1957-58 Apr. 1958 Winter 1956-57 Mar. 1963 -

60 60 60 55-69 62 62 64 62-69 68

215 220 210 210

k 2 & 10 f 10 f 25 275 (4) 210 (4) 280 f 20 (2) 215 rt 15 >300 (2)

1964 1960 1953 1964 1964

Sep.-Nov. 1963 Mar. 1963 Feb.-Mar. 1957 Nov. 1952 Mar. 1964 Mar. 1963

70 69-77 75

210 170 275 300 185 160

Jan. 1959 ;u&J;t& 1959

For references see Wallace (1960). Corrected values given by Wallace (1960). Corrected value given by G. Kvifte (1961). Corrected by the author following Wallace (1960).

-16 10-20

Rot. temp. corr. to winter (“K)

Rot. temp. (“K)

6?85 77-85

-f 10 * 15 f 35 (2) f 25 (2) f 25 -+ 15

225 230 235 235 275 235 280 245 >300 240 215 310 315 230 205

-

225 f 5

230 f 5

250 f 10

255 f 20

1520

G. J. KVIFTE

the spread of the extrapolated temperatures is less than of those directly recorded. We still have, however, a branching out at high latitudes, one branch showing high, the other one low temperatures. From Table 2 we note that the low temperatures all are obtained during the years 1963-64 while the high ones mostly are found during the period 1956-58. As these periods are the period of sunspot minimum and maximum respectively, this may reflect a variation 330

.

.

.

.

4 0 I-_-

l: .

210

.

.:

-

a9 .

.

.

I :

.

.

.

. .

150

e

. la’

c

270

I

I

Ib)

150

0

15

30

15

Latitude,

60

75

90

ON

1. OH TEMPERATURESVS. LATITUDE. (a) original data 1949-1964 (See Table 2 for references); (b), temperatures extrapolated to winter solstice. FIG.

of the atmospheric temperature with the solar cycle. This effect is well established at heights above 120 km (CIRA, 1965), but lack of data has up to now prevented any investigation of this effect at lower heights. As the OH temperatures show much less scatter at low latitudes, this effect may be restricted to the polar regions at OH heights. Knowing the extremum values of the variation of the OH temperature with the solar cycle, the temperature at middle activity should be obtained by some averaging procedure, depending on the variation between the times of maximum and minimum. This variation being unknown, however, the mean temperature at all latitudes will be given, as shown in the last column of Table 2, as the simple average of all available data within intervals of 10” latitude. As is seen, a temperature profile is obtained, showing increasing values from 225°K

NIGHTGLOW

ON ROTATIONAL

TEMPERATURES

AND INTENSITIES

1521

at 40-50”N to 255°K at 70-80”N. The equator observations are too few to be given much weight at present and are omitted. This dependence with latitude agrees reasonably well with the corresponding variation of the atmospheric mean temperature at 100 km (Fig. 2). Thus our assumption of an OH emission height of about 100 km seems plausible. One might argue that assuming some other height of the OH emission, when correcting the original data to a common time of the year, might give this new height when comparing the averaged corrected values with the atmospheric temperature variation with latitude. Assuming heights more than a few kilometers apart from the lOO-km level will, however, contradict the assumption of equivalence of the atmospheric and the OH temperature. Moreover, using other heights in the computations will still give a resulting height near 100 km.

110 km

_

105 km

100 km 95km 90km

0

15

45

30 Latitude,

60

75

90

lN

FIG. 2. AVERAGEDWINTERTEMPERATURES vs. LATITUDE.

The curves show the dependence

of the atmospheric temperature Murgatroyd (1965).

with latitude as given by

Looking at the variation of solar activity, represented by the solar 10*7-cm flux (CIRA, 1965), an extreme increase is observed during the winter 1957-58. Hence the assumption of increasing atmospheric temperature with solar activity at OH heights may explain also the great winter maximum in OH temperatures this winter, as found by Krassovsky et al. (1961). 5. BAND

INTENSITIES

To obtain the total band intensities also the lines which are too faint to be observed or are contaminated by other emissions have to be included. This may be done by using equation (l), which gives relative intensities of the individual lines within a band. From this equation relative intensities within bands with upper vibrational level u = 5 have been computed by Krassovsky et al. (1962) in the temperature range 150-330°K. According to Chamberlain (1961) these intensities may also be used for the bands observed here. Band intensities derived in this manner are given in rayleighs, reduced to zenith values, in Table 1. The relative intensities from As and Tromso are supposed to be accurate within 10 and 15 per cent, respectively, while the absolute values may be systematically displaced as much as 50 per cent from their true values, due to calibration errors.

G. J. KVIFTE

1522

5.1. Seasonal variation in band intensities As the different bands are observed only a few times each, it is difficult directly to find any seasonal variation of the OH intensity. This effect is therefore investigated by giving the intensity of each band relative to the typical values given by Krassovsky et al. (1962). The plots of these ratios with time are shown in Fig. 3. A slight decrease with time may be seen during the fall at Tromsra 1963, while a marked increase is seen during the following spring at As. This indicates, in contrast to earlier work, a minimum of the intensity during winter. At both stations the winter minimum probably will show intensities near O-5of those given by Krassovsky et al. (1962). Symbol Band

l

9-L

+

o

8-3

6-1

q

9-3

Y 8-2 +c +,

0

0

‘ r

-rT

I

0

-

1

:T

P S

0

D

N

J

F

M

A

FIG. 3. OH BAND INTENSITIES FROM NORWAY 1963-64. The autumn values are obtained at Tromss and the spring values at As. The intensities are given relative to the mean values from Krassovsky et al. (1962).

q q

Intensities

by

Intensities

from

Krassovsky

et

al

(1962)

Trams6

800

0 9-L FOG.

4.

AVERAGED

8-3

6-l

9-3

INTENSITIES OF OH BANDS OBTAINED FROM NORWAY TO MEAN VALU@S FROM KRASSOVSKYet af. (1962).

8-2

1963-64

COMPARED

5.2. Latitude variation in band intensities As the observations from Tromso and As seem to give an intensity minimum of the same magnitude at the two stations, any variation of the OH intensity with latitude between 60”N and 70”N should be rather moderate. However, the variation with season found from Tromss should be considered with some reservation in view of the large scatter of the individual observations there. Hence a better way to investigate the dependence with latitude may be as shown in Fig. 4, where the averaged intensities of each observed band are given.

NIGHTGLOW

OH ROTATIONAL

TEMPERATURES

AND INTENSITIES

1523

The figure also shows the typical mean intensities given by Krassovsky et al. (1962). As is seen, their values are in good agreement with the As results. Supposing this to be true as well for the 8-2 and 9-3 bands, which are observed at Tromso only, their values may be used as approximate intensities at As for these bands. As a result we see the intensities from Tromso to be at most a factor two weaker than the As results. Thus any latitude effect should be small between 60”N and 70”N. Acknowledgements-The author is indebted to Prof. A. Omholt and Prof. G. Kvifte for valuable discussions and to Mr. H. Pettersen and Mr. 0. Harang for kind assistance during the work. I also wish to express my thanks to the staff at the Dept. of Physics, Agricultural College of Norway, As, and of the Aurora1 Observatory, Tromso. This research is sponsored in part by the Norwegian Research Council for Science and the Humanities by grant D183-9. REFERENCES BAKERD. J. (1966). Final Rep. AFl9(628)-251, Utah State University. BARBIER D. (1957). C.r. hebd. Sdanc. Acad. Sci., Paris 245, 1559. BARBIER D. (1959). AnnIsgPophys. 15,412. BLACKWELL D. E., INGHAMM. F. and RULE H. N. (1960). Asfrophys. J. 131,15. CABANNES J.. DUFAY J. and DUFAY M. (1950) C.r. hebd. Sganc. Acad. Sci.. Paris 230. 1233. CHAMBERLAIN J. W. (1961). Physics of the Aurora and Airglow, p. 556. Academic Press, New York. CHAMBERLAIN J. W. and MEINELA. B. (1954) (Ed. Kuiper) The Earth as a Planet, p. 514. University of Chicago Press.

CHAMBERLAIN J. W. and OLIVERN. J. (1953). Phys. Rev. 90, 1118. CIRA

(1965).

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Amsterdam.

DUFAY J. and D~JFAYM. (1951). C.r. hebd. SLanc. Acad. Sci., Paris 232,426. DUFAY M. (1959). Annkgkophys. 15, 134. GUSH H. P. and VALLANCEJONESA. (1955). J. atmos. terr. Phys. 7,285. HP~NLH. and LONDONF. (1925). Z. Phys. 33,803. KRA~~OVSKY V. I., SHEFOVN. N. and YARIN V. I. (1961). J. atmos. terr. Phys. 21,46. KRASSOVSKY V. I., SHEFOVN. N. and YARIN V. I. (1962). Planet. Space Sci. 9, 883.

KWTE G. (1960). KWFEJ G. (1961).

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Agricult. College, Norway.

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MCPHERSON D. H. and VALLANCEJONESA. (1960). J. atmos. terr. Phys. 17, 302. MEINELA. B. (1950a). Asrrophys. J. 111, 555. MEINELA. B. (1950b). Astrophys. J. 112, 120. MIRONOVA. V., PRODUKINA,V. S. and SHEFOVN. N. (1958). Annlsg&ophys. 14, 364. MURGATROYDR. J. (1965). Tech. note 70, World Meteorological Organization. 199. NAKAM~JRA M. (1961). Rep. Zonosph. Space Res. Japan 15, 346. NGUYEN-HUU-DOAN(1963). Annlsgkophys. 19, 406. NOXON J. F. (1964). J.geophys. Res. 69, 4087. OMHOLTA., STOFFREGEN W. and DERBLOMH. (1962). J. atmos. terr. Phys. 24,203. PACKERD. M. (1961). An&. gkophys. 17, 67. WALLACEL. (1960). J. geophys. Res. 65, 921. WALLACEL. (1961). J. atmos. terr. Phys. 20, 85. WALI~ACE L. (1962). J. atmos. Sci. 19, 1. PeaIe!W+-CneKTpbI pHHa AIranaaoHoB ~MHCCHIIHOYHO~OCReHeIIHH011 6bInH nonyHeHbI B Tpo~aii OCeHbIo 1963 rona 14 B hce BeCHOIO 1964 rona. PanMoHanbHbIe TeMnepaTypbI 6bIHH HOCTH~HYT~Ico CpeHHel BenHsnHoti B 210 * 10°K na 060~~ CTaHqaHx. 3aBHcHMOCTH OT BpeMeHM rOHa ElJlH IIIHPOTbI 06Hapy?KeHO He 6bmo. TeMIIepaTypbI OH, IIOJIy~eHAbIe

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